MySQL 8.0新特点 CTE

2024-07-24 12:36:21

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MySQL 8.0新特点 CTE：

　　1、CTE简介

　　MySQL从8.0开始支持CTE，慢慢地向Oracle学习，CTE确实是个很好用的东西，特别是针对OLAP类型的SQL,可以大大简化，优化SQL.

　　那么什么是CTE呢？

　　个人理解：CTE(common table expression)是一个临时的结果集，类似一个函数，一旦定义好，可以多次调用。

　　2、CTE语法

　　with_clause:
　　    WITH [RECURSIVE]
　　        cte_name [(col_name [, col_name] ...)] AS (subquery)
　　        [, cte_name [(col_name [, col_name] ...)] AS (subquery)] ...
　　(1)列别名可以在不同的位置定义

　　mysql> WITH cte (col1, col2) AS
　　    -> (
　　    ->   SELECT 1, 2
　　    ->   UNION ALL
　　    ->   SELECT 3, 4
　　    -> )
　　    -> SELECT col1, col2 FROM cte;
　　+------+------+
　　| col1 | col2 |
　　+------+------+
　　|    1 |    2 |
　　|    3 |    4 |
　　+------+------+
　　2 rows in set (0.00 sec)

　　等价与：
　　mysql> WITH cte AS
　　    -> (
　　    ->   SELECT 1 AS col1, 2 AS col2
　　    ->   UNION ALL
　　    ->   SELECT 3, 4
　　    -> )
　　    -> SELECT col1, col2 FROM cte;
　　+------+------+
　　| col1 | col2 |
　　+------+------+
　　|    1 |    2 |
　　|    3 |    4 |
　　+------+------+
　　2 rows in set (0.00 sec)
　　(2) CTE用在Select操作

　　mysql> create table t1(a int,b int);
　　mysql> insert into t1 values(1,1),(2,2),(3,3);
　　mysql> with t as
　　       (select a+2 c,b from t1)
　　       select c,b from t;
　　+------+------+
　　| c    | b    |
　　+------+------+
　　|    3 |    1 |
　　|    4 |    2 |
　　|    5 |    3 |
　　+------+------+
　　3 rows in set (0.00 sec)
　　(3)CTE用在DML操作

　　mysql> with t as (select a+2 as a,b from t1) update t1,t set t1.a=t.a+10 where t1.a=t.a;
　　mysql> select * from t1;
　　+------+------+
　　| a    | b    |
　　+------+------+
　　|    1 |    1 |
　　|    2 |    2 |
　　|   13 |    3 |
　　+------+------+
　　3 rows in set (0.00 sec)

　　mysql> with t as (select a+2 as a,b from t1) delete t1 from t1,t where t1.a=t.a;
　　mysql> select * from t1;
　　+------+------+
　　| a    | b    |
　　+------+------+
　　|    1 |    1 |
　　|    2 |    2 |
　　+------+------+
　　2 rows in set (0.00 sec)

　　mysql> insert into t1 with t as (select 10*a as a,b from t1) select * from t;
　　mysql> select * from t1;
　　+------+------+
　　| a    | b    |
　　+------+------+
　　|    1 |    1 |
　　|    2 |    2 |
　　|    3 |    3 |
　　|   10 |    1 |
　　|   20 |    2 |
　　|   30 |    3 |
　　+------+------+
　　6 rows in set (0.00 sec)
　　3、CTE可以优化SQL

　　(1)下面第一条SQL可以改写成如下两种CTE简化形式

　　mysql> select count(*) from employees e1
　　       left join (select * from employees) e2 on e1.emp_no=e2.emp_no
　　       left join (select * from employees) e3 on e2.emp_no=e3.emp_no;
　　

　　mysql> with e2 as (select * from employees),
　　            e3 as (select * from employees)
　　            select count(*) from employees e1
　　            left join e2 on e1.emp_no=e2.emp_no
　　            left join e3 on e2.emp_no=e3.emp_no;
　　
　　mysql> with e as(select * from employees) select count(*) from employees e1
　　        left join e e2 on e1.emp_no=e2.emp_no
　　        left join e e3 on e2.emp_no=e3.emp_no;
　　(2)CTE的本质是子查询，所以子查询的一些特性都适用，如子查询合并。

　　mysql> desc with e as(select /*+ set_var(optimizer_switch='derived_merge=off')*/ * from employees)
　　    -> select count(*) from employees e1
　　    -> left join e e2 on e1.emp_no=e2.emp_no
　　    -> left join e e3 on e2.emp_no=e3.emp_no;
　　+----+-------------+------------+------------+-------+---------------+-------------+---------+------------------+--------+----------+-------------+
　　| id | select_type | table      | partitions | type | possible_keys | key         | key_len | ref              | rows   | filtered | Extra       |
　　+----+-------------+------------+------------+-------+---------------+-------------+---------+------------------+--------+----------+-------------+
　　| 1 | PRIMARY     | e1         | NULL       | index | NULL          | PRIMARY     | 4       | NULL             | 299512 |   100.00 | Using index |
　　| 1 | PRIMARY     | <derived2> | NULL       | ref   | <auto_key0>   | <auto_key0> | 4       | testdb.e1.emp_no |     10 |   100.00 | NULL        |
　　| 1 | PRIMARY     | <derived2> | NULL       | ref   | <auto_key0>   | <auto_key0> | 4       | e2.emp_no        |     10 |   100.00 | NULL        |
　　| 2 | DERIVED     | employees | NULL       | ALL   | NULL          | NULL        | NULL    | NULL             | 299512 |   100.00 | NULL        |
　　+----+-------------+------------+------------+-------+---------------+-------------+---------+------------------+--------+----------+-------------+
　　4 rows in set, 2 warnings (0.00 sec)
　　(3)CTE可以起到减少插入临时表数据，优化SQL的作用

　　mysql> flush status;
　　Query OK, 0 rows affected (0.02 sec)
　　mysql> select /*+ set_var(optimizer_switch='derived_merge=off')*/ * from
　　    -> (select * from t_group) t1
　　    -> join (select * from t_group) t2
　　    -> on t1.emp_no=t2.emp_no;
　　+--------+---------+------------+------------+--------+---------+------------+------------+
　　| emp_no | dept_no | from_date | to_date    | emp_no | dept_no | from_date | to_date    |
　　+--------+---------+------------+------------+--------+---------+------------+------------+
　　| 22744 | d006    | 1986-12-01 | 9999-01-01 | 22744 | d006    | 1986-12-01 | 9999-01-01 |
　　| 24007 | d005    | 1986-12-01 | 9999-01-01 | 24007 | d005    | 1986-12-01 | 9999-01-01 |
　　| 30970 | d005    | 1986-12-01 | 2017-03-29 | 30970 | d005    | 1986-12-01 | 2017-03-29 |
　　| 31112 | d002    | 1986-12-01 | 1993-12-10 | 31112 | d002    | 1986-12-01 | 1993-12-10 |
　　| 40983 | d005    | 1986-12-01 | 9999-01-01 | 40983 | d005    | 1986-12-01 | 9999-01-01 |
　　| 46554 | d008    | 1986-12-01 | 1992-05-27 | 46554 | d008    | 1986-12-01 | 1992-05-27 |
　　| 48317 | d008    | 1986-12-01 | 1989-01-11 | 48317 | d008    | 1986-12-01 | 1989-01-11 |
　　| 49667 | d007    | 1986-12-01 | 9999-01-01 | 49667 | d007    | 1986-12-01 | 9999-01-01 |
　　| 50449 | d005    | 1986-12-01 | 9999-01-01 | 50449 | d005    | 1986-12-01 | 9999-01-01 |
　　| 10004 | d004    | 1986-12-01 | 9999-01-01 | 10004 | d004    | 1986-12-01 | 9999-01-01 |
　　+--------+---------+------------+------------+--------+---------+------------+------------+
　　10 rows in set (0.00 sec)
　　mysql> show status like '%handler_write%';
　　+---------------+-------+
　　| Variable_name | Value |
　　+---------------+-------+
　　| Handler_write | 20    |
　　+---------------+-------+
　　1 row in set (0.00 sec)

　　mysql> flush status;
　　Query OK, 0 rows affected (0.02 sec)
　　mysql> with t as(select /*+ set_var(optimizer_switch='derived_merge=off')*/ * from t_group)
　　    -> select * from t t1
　　    -> join t t2 on t1.emp_no=t2.emp_no;
　　+--------+---------+------------+------------+--------+---------+------------+------------+
　　| emp_no | dept_no | from_date | to_date    | emp_no | dept_no | from_date | to_date    |
　　+--------+---------+------------+------------+--------+---------+------------+------------+
　　| 22744 | d006    | 1986-12-01 | 9999-01-01 | 22744 | d006    | 1986-12-01 | 9999-01-01 |
　　| 24007 | d005    | 1986-12-01 | 9999-01-01 | 24007 | d005    | 1986-12-01 | 9999-01-01 |
　　| 30970 | d005    | 1986-12-01 | 2017-03-29 | 30970 | d005    | 1986-12-01 | 2017-03-29 |
　　| 31112 | d002    | 1986-12-01 | 1993-12-10 | 31112 | d002    | 1986-12-01 | 1993-12-10 |
　　| 40983 | d005    | 1986-12-01 | 9999-01-01 | 40983 | d005    | 1986-12-01 | 9999-01-01 |
　　| 46554 | d008    | 1986-12-01 | 1992-05-27 | 46554 | d008    | 1986-12-01 | 1992-05-27 |
　　| 48317 | d008    | 1986-12-01 | 1989-01-11 | 48317 | d008    | 1986-12-01 | 1989-01-11 |
　　| 49667 | d007    | 1986-12-01 | 9999-01-01 | 49667 | d007    | 1986-12-01 | 9999-01-01 |
　　| 50449 | d005    | 1986-12-01 | 9999-01-01 | 50449 | d005    | 1986-12-01 | 9999-01-01 |
　　| 10004 | d004    | 1986-12-01 | 9999-01-01 | 10004 | d004    | 1986-12-01 | 9999-01-01 |
　　+--------+---------+------------+------------+--------+---------+------------+------------+
　　10 rows in set, 1 warning (0.00 sec)
　　mysql> show status like '%handler_write%';
　　+---------------+-------+
　　| Variable_name | Value |
　　+---------------+-------+
　　| Handler_write | 10    |
　　+---------------+-------+
　　1 row in set (0.00 sec)
　　CTE除了一般功能外,还可以实现递归一些复杂SQL需。

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